Spiro-Based Thermally Activated Delayed Fluorescence Emitters with Reduced Nonradiative Decay for High-Quantum-Efficiency, Low-Roll-Off, Organic Light-Emitting Diodes

Herein, we report the use of spiro-configured fluorene-xanthene scaffolds as a novel, promising, and effective strategy in thermally activated delayed fluorescence (TADF) emitter design to attain high photoluminescence quantum yields (φPL), short delayed luminescence lifetime, high external quantum efficiency (EQE), and minimum efficiency roll-off characteristics in organic light-emitting diodes (OLEDs). The optoelectronic and electroluminescence properties of SFX (spiro-(fluorene-9,9′-xanthene))-based emitters (SFX-PO-DPA, SFX-PO-DPA-Me, and SFX-PO-DPA-OMe) were investigated both theoretically and experimentally. All three emitters exhibited sky blue to green emission enabled by a Herzberg-Teller mechanism in the excited state. They possess short excited-state delayed lifetimes (<10 μs), high photoluminescence quantum yields (φPL ∼70%), and small singlet-triplet splitting energies (ΔEST < 0.10 eV) in the doped films in an mCP host matrix. The OLEDs showed some of the highest EQEs using spiro-containing emitters where maximum external quantum efficiencies (EQEmax) of 11 and 16% were obtained for devices using SFX-PO-DPA and SFX-PO-DPA-OMe, respectively. Further, a record EQEmax of 23% for a spiro-based emitter coupled with a low efficiency roll-off (19% at 100 cd m-2) was attained with SFX-PO-DPA-Me.